"I really don't want to be in an elevator alone with her," McCaskill told a friend in remarks quoted in the forthcoming "HRC: State Secrets and the Rebirth of Hillary Clinton."
The tension between the two Democrats came after McCaskill said on NBC's "Meet the Press" while she was running for election that former President Bill Clinton was "a great leader, but I don't want my daughter near him," authors Jonathan Allen of Politico and Amie Parnes of the Hill say in the 448-page book, to be published Feb. 11.
McCaskill instantly regretted the remark, the book says.
Indeed, the unfiltered comment brought her "to the point of epic tears," the book quotes a McCaskill friend as saying.
The remark prompted Hillary Clinton to immediately cancel a planned fundraiser for McCaskill, the book says.
McCaskill phoned Bill Clinton a few days later to give a tearful apology, and his gracious response only deepened her distress, the book says.
The book also describes a record drawn up by Clinton aides that listed lawmakers and convention superdelegates who betrayed the candidate during the 2008 primary, the Hill reported.
The book describes how the list ranked lawmakers numerically on how helpful they had been during the primary.
One of the representatives who was among the worst ranked, Rep. Chris Van Hollen, D-Md., said Monday he doesn't know why he was reported as being on the Clinton political hit list, saying he's trying to figure out what -- if anything -- he did.
"I'm trying to figure out this last one," he said on MSNBC. "As I say, it's an unsolved mystery and maybe someone will pick up the phone and tell me. Again, this assumes that this is a true report."
Van Hollen, who led the Democratic Congressional Campaign Committee in 2008, said he's a big Clinton supporter and that he hopes she seeks the Democratic presidential candidacy in 2016.
"I hope she will run for president," he said. "And I've indicated, I would be a supporter of Hillary Clinton, if she runs."
In 2007, when Hillary Clinton, then a U.S. senator from New York, started her unsuccessful White House run, she sought to mend fences with McCaskill at a private lunch in the Senate dining room, where the two spoke about the physical rigors of campaigning.
But the bonding attempt didn't stop McCaskill from endorsing Sen. Barack Obama, D-Ill., for president in January 2008.
McCaskill, however, appears to be trying to improve relations with Hillary Clinton -- most conspicuously with her June 2013 endorsement of Clinton for president in 2016.
"Now, as I look at 2016 and think about who is best to lead this country forward, I'm proud to announce that I am Ready for Hillary," McCaskill said a statement posted at ReadyforHillary.com.